Thanks for your help! Such a script would be really cool and a help to every photographer!

Edit: When I think about it, it would be nice to rename the files i a first step by defining a name like "monday" with the result, that the photos will have filenames like "monday-001.jpg, monday-002.jpg, monday-003.jpg" etc

This might not be exactly what you want, but: have you tried The "Script-Fu" in GIMP? I'm pretty sure you could write your own script there that only saves the image once at the end.Don't know about the renaming/moving stuff, but I guess if GIMP cannot do that, you could always have a bash script do all of that in a second step.

I've only used it once myself for a purpose similar to yours. Although I had no experience at all it didn't take me more than an hour.

I just opened an image in GIMP, clicked on Filters > Script-Fu > Console, and used the "Browse..." button to find out the names of the functions I needed (e.g. "gimp-image-resize"). Then I googled the functions to find out the correct syntax for my script.